Analog to digital converter



June 7, 1960 R. L. KINDRED ANALOG To DIGITAL CONVERTER 3 Sheets-Sheet 1 Filed Jan. 16, 1956 June 7, 1960 R. l.. KINDRED ANALOG To DIGITAL CONVERTER 3 Sheets-Sheet 2 Filed Jan. 16, 1956 u mm. .1

A vvvvvvvv vvvvvvvv INVENTOR.

R.L.KINDRED ATTORNEYS June 7, 1960 R. L. KINDRED ANALOG To DIGITAL CONVERTER 3 Sheets-Sheet 3 Filed Jan. 16, 1956 INVENTOR.

R. L. KINDRED ATTORNEYS United States Patent O 2,940,071 ANALOG To DIGrrAL CONVERTER Raymond L. Kindred, Bartlesville, Okla., assignor to .Phillips Petroleum Company, a corporation of Delaware Filed Jan. 16, 1956, Ser. No. 559,170

13 Claims. (Cl. 340--347) This invention relates to apparatus for converting a voltage into a digital representation.

In various fields of automatic control, a variable to be measured is represented by an analog voltage through the use of appropriate transducing equipment. The final data processing step often involves the use of electronic digital computers. Thus, it is necessary to provide apparatus for converting analog voltages into corresponding digital representations. Such converters are also useful in observing physical measurements. For example, the output signal from a conve-rter of this type can illuminate a series of lamps so that a voltage to be measui'ed isA represented in a digital fashion by the illuminated lamps. This relieves the operator from the task of interpreting data and often results in a more accurate measurement. j Y In accordance with the present invention apparatus is `provided to express analog voltages in digital form. A self-balancing 'potentiometer principle of voltage measurement is' employed in this instrument. A reference voltage is applied across the potentiometer and the voltageat the contactor is compared with vthe voltage to be measured. The difference between these voltages is utilize'd to control the setting of the potentiometer contactorl The potentiometer lis in the form of a'plurality of voltage dividingv networks which areassociated with a1 plurality;y offsteppingl switches. The output of thevoltagecomparing means is applied through appropriate relaysto energize selected ones'of thel stepping relays until the two voltages are equal. A series of lamps can be associated with the stepping switches so that the voltage can be-measu'red is represented inv a digitalfashion by` selected lamps being illuminated Accordingly, it isv an object of this invention to provide apparatus for expressing a voltage to be measured in digital form. t

Another Objectis to provide apparatus for convertingI analog voltages into signals suitable for introduction into: digital computers.

Other objects, advantages and features of the invention should become apparent from the following detailed description taken in conjunction wtih the accompanying drawing in which:

Figure 1 is a schematic representationv of a present preferred embodiment of the converter of this invention;

Figure 2'is a more detailed circuit diagram of the voltage comparison circuit of Figure 1; and

Figure 3' is a more detailedschematic circuit of the .stepping switches and relays of Figure l.

InA a conventional. null ybalance servomechanism, the feedback potentiometer is continuously variable and is driven =by a motor or other device which is capable ofv notcontinuous. This means that a voltage op shaft positon repesentingai number must' take on definite valuesy lCC 2 in steps. The apparatus of the present invention utilizes a series or" stepping switches which are unidirectional in operation. In apparatus of this type the settings of the switches must be accomplished without a reversal of the switching direction.

In Figure 1 there is shown a schematic representation of three stepping switches 11, 12 and 13. These switches represent the hundreds, tens and units of the desired digital representation. A first voltage dividing network 15 is vassociated with stepping switch 11. This'` network comprises ten resistors 16 which are connected in series relationship with a source of reference potential (Vm) 17. Eleven contacts 18 of stepping switch 11 are connect'ed to the junctions between resistors 16. The upper ten of these contacts are designated by the' numerals O,Vv l, 2 .f 9. Stepping switch 11 is provided with a pair of contacts 11b and 11i,` which are mechanically coupled 4to one another and which rnove in unison to engage respective adjacent contacts 18 when the steppingl switch is energized.

A second voltage dividing network 21 is associated with stepping switch 12. Network 2.1 comprises ten resistors 22 which are connected in series relationship. Contacts 2.3 are connected to the junctions between these resistors. The iirst end terminal of network 21 is connected to contact 11C of switch 11 and the second end terminal of network 211 is connected to contact 11b of switch 11. Switch 12 is provided with a pair of contacts 12b and 12C which are mechanically connected to one another and which move to engage adjacent pairs ofy contacts 23 when switch 12 -is energized. Switches 12b andv 12C move -in a direction opposite the direction of the' movement of switches 11b and 11e so as to engage in sequence the contacts 23 which are designated by numerals 9, 8, 7 0.

A third voltage dividing network Z7 4is associated with stepping switch 13. This network comprises ten' resistorsv 28, which are connected' in series relationship. The rs't end yterminal of network 27 is connected to contact 12e and the second end terminal of network 27 *isv connected `to Contact 12b. A series of contacts! 29' are-connected to the junctions between resistors 2li. Stepping switch 13 is provided with a single switch 13C which moves" to engage the contacts designa-ted by the numerals O, 1,l Z 9 in sequence when the stepping switchl is energized.

The direct current analog voltage to beV measured'` (Vs) is applied between input terminals 33 and 34. One of these input terminals is` connected to ground. Terminal 34 is connected to the negative terminal of potential source 17 and to the rst end 4terminal of net-v work 15. Terminal 33 is connected to the iirst input terminal of a chopper 3S which is energized at a predetermined frequency by an oscillator 36. The second input terminal of chopper 35 is connected to a relay switch' 37. Switch 37 engages a first contact in its up position which is connected to the uppermost terminal' 9 of stepping switch 13. Switch 37 engages a second contact in its down position which is connected to switch 13o of stepping switch 13. Throughout the following description the relay switches will `be described as occupying upper or lowerv positions. The contacts engaged by these switches in the twoV positions will be referred ltoas the upper and lower contacts'. This terminology is employed merely to simplify the description.

The output signal ofchopperY 35 is applied to the input terminal of an amplifierv 38 which is tuned to pass signals of the frequency of oscillator 36. The output terminals of oscillator 36 andamplilier 38 are connected to the respective inputY terminals of a phase detector I40. The first output terminal4 ofphase detector 40 is connected through the coil of a positive error relay 41 t and aA rectifier 42to the second output telminal of phase considered as `N'.

3 detector 40. The second output terminal of phase detector 40 is connected through `a rectifier 43 and the coil of Ia negative error relay 44 to a switch 45. The lower contact of switch 45 is connectedtothe rst output terminal of phase detector 40. A

Stepping switches 11, 12 and 13 are energized from a current source 47. One termin-al of source 47 is connected to grou-nd, as are first terminals of the stepping switches. The second terminal ofsource .47 is connected through a switch 48 to a switch 49 which is actuated by relay 44. The upper contact of switch 49 is connected to a switch 50 which is -actuated by relay 41. The lower contactrof switch V50 is connected 4to a switch 51. The upper contact of switclrSlV is connect-ed to` stepping switch 11 and the lower contact of switch 5l is connected to stepping switch 13V. The lower contact of switch 49,7is connected to stepping switch 12.*V Fl'he upper contactY of switch 50 is connected to the upper contact of a switch 53. Switch 53 is connected to ground through the coil of a carry-around relay 54. A capacitor 55 is connected in parallel with the coil of relay 4. The lower contact of switch 53 `is connected through a switch 56a to the first terminal of acurrent source 57. The second terminal of source 57 is connected to ground. The iirst terminal of source 47 is connected through respective switches 5613, 56C and 56d to stepping switches 13, 12 and 11, respectively. Switches 56a, 56h, 56C and 56d are elements of switch 56 and are operated in unison. Switches 45, 37, 51 and 53 are actuated in unison by relay 54. The various switches `occupy theA illustrated positions in the absence of current being suppliedto the associated relays.

. In explaining the operation of the apparatus thus far described it will be assumed that the `initial value of the feedback voltage applied to chopper-35 is 0.099 fof the reference Vvoltagersupplied by 'source .17. This value corresponds to the stepping switches occupying the illustrated positions. is greater than the :feedback voltage, a .positiveV output signal lis provided by phase `detector 40 which encrgizes relay 4 1. vWhen switch 48 is closed to start the operation the stepping current fromr source 47 is applied through the upper contact of switch 49, the lower contact of switch 50 and the uppercontactfofswitch 51 tostepping switch 1'1. 1Stepping`switch 11 continues to I If the signal voltage to be measured` be actuateduntil the feedback voltage is equal t-o 0N99 ofthe referencevoltage. This value is equal to or greater than the signal voltage to be measured. L The error volt-A age (Ve)V thus becomes zero'or negative soV that relay 41 is deenergized. ,At this time source 47 fis connected through theupper contacts of .switch .49, and 53 to relay54 so that this relay is energized. This moves switch 37 to the .lower positionV so that the feed-back voltage becomes 0.N90 of the reference voltage.V This value can be either greater or lessthan the signal voltage. In Athe more general case, the feedback voltage is greater than the signal voltage so that the error voltagel isnegative and relay 44 is energized. AIt should be noted that this relay cannot become energized luntil relay 54 is energized to close switch 45. The reason -for this is explained hereinafter. When relay 44 is energized source 47 is connected directly to stepping switch l2 which causes Vswitch 12bV to move.v downwardly to a contact The feedback voltage is then 0.NN0 ofthe referenceA voltage. This value can 'bek either equal to or less than the signal voltage. I-f the feedback voltage is equal to the, signal voltage the process -is complete. However, in the more general case the feedback voltage again becomes less than the signal voltage so that 'relay 41is'o'nce' againfenergized. At this time source?, 47 is applied to steppingV switch' 13 because switch 51l is'.

in engagement with its lower contact. V.Switch 13a` or between capacitor 88 and resistor 89. The cathode of...

4 of switches 11b, 12b and 13e thus represent the .signal voltage in digital form which is correct to three signlcant figures.

If the signal voltage is initially equal to or less than 0.099 of the reference voltage, the carry-around relay 54 is energized as soon as switch 48 is closed. The setting procedure is then the same as previously described. If the signal voltage has the value of 0.N9N, the orlglnal.- error voltage is positive when carry-around relay 54 be-r comes energized so that stepping switch 12 does not" become actuated. vStepping switch 13' operates in the manner previously described.

The stepping relays remain in the linal positions until -the reset mechanism is actuated by switch 56. This. closes switches 565, 56e and'Sod and opens switch 56a. Current source 47 is then .applied to the reset mechanism of the three stepping switches to move the switch arms back to the initial positions. y l

The voltage comparing circuit of Figure l is illustrated in detail in Figure 2. Input terminal 33 is connected to `the upper contact of a switch 70.` Switch V7i) is connected to the yrst stationary terminal 71 of chopper 35. The lower terminal of switch 70 is connected to the first terminal of a voltage source 72. The second terminal of voltage source 72 is connected to ground. Voltage source 72 is provided to calibrate the instrument in the manner described in greater detailhereinafter. VInput terminal 34 is connected to .a switch 73 and to a terminal 7,4 which is connected to the voltage dividing network illustrated in Figure 3.

chanicallycoupled and normally occupy the illustrated up positions so that source 72 is removed from` the circuit.Y The second stationary terminal 75 of 'chopper 35 isvconnected'to a terminal 76 which also is connectedl to the control `grid of a triode .81 which vforms the SrstA stage ofV :amplifier `38. n 'Ihe cathode of triode a resistor 82. Theanodeoof triode 31 is connected to a positiveV potential lterminal. through series connected resistors 84 and 85. .A capacitoro is connected between ground. andthe junction-(between resistors 84 and 8,5. The .anode of triode 81 is connected to the control grid of a second'triode 87 through a capacitor 88 and resistor 89 which are connected in series `relationship. kA resistor 190 is connected between ground and the junction triode 87 is connected directly to ground. The anode of triode 87 is connected to terminal V83 through series connected resistors 92 and `93|. A capacitor 94 is connected between ground and the junction between these resistors. The anode of tricde 87 istconnected to ground through a capacitor 95 anda potentiometer 96` which are connected in series relationship. Y The contactor ot'v potentiometer 96 is connected to the control grid of a third triode 97 Vthrough a resistorr98.

VThe cathode of triode` V97v is connected directly' to y ground. Ute anode of triode'rl is Yconnected to terminal 83 through series connected resistors 180 and 101.

A capacitor 102 is connectedl between ground andthe f junction between theseresistors.` KFlhe ancdeof triode 97 is connected to the control grid of a fourth triodelt, through a `capacitor 104 and a resistor 105 which are Y The lower contact of switchv 73, is connected to ground. Switchesv70 and 73 are 1ne-.

is` connected tor ground through` The anode of triode 103 is connected to ground lthrough a capacitor `108 and a potentiometer 109. The contactor of Apotentiometer 109 is connected 4to the control grid of a fifth triode 111 through series .connected resistors 112 and 113.

'Ilhe circuit thus far described comprises a three-stage clipper amplifier with a pre-amplifier. Resistors 89, 98 and are provided to reduce the amplitude of the transmitted signal so that the signal applied to the control grid of triode 111 is independent of the amplitude of the input signal within rather wide limits. The output signal of amplifier 38 is of a phase which depends upon the relative amplitudes of the two signals applied to chopper 35. It is desired that this signal be of substantially constant amplitude in order that the phase'thereof can readily be detected by phase detector 40. The various coupling resistorsare fairly large to provide the desired clipping. The signal at the arm of potentiometer 1'09 is substantially of square wave form and is of the same frequency as the signal applied to coil 80 of chopper 35.

anode of triode 117 is connected to terminal 83 through Y a resistor 122. The anode of triode 117 is connected to ground through a capacitor 123 and the primary winding 124 of a transformer .125. Transformer 125 provides the first input of phase detector 40. n A' A parallel-T filter 127 is connected in series relationship with a capacitor 128 between the control grid 'of triode 117 and the control grid of triode .111. 1 Filter 127 comprises series connected resistors 130 and131 con'- nected in parallel with 'series connected capacitors 132 and 133. The junction between'resistors 130 and 131 is connected to ground through a capacitor `134. The

- junction betweengcapacitors 132 and 133 is connected to ground through a resistor 135. Filter 127 is tuned to the frequency of the signal transmitted by amplifier 38. This results in the output signal from the amplifier having substantially a sinusoidal wave form. f l

Oscillator 36 can advantageously comprise a Wein bridge oscillator of the form Villustrated in Figure 2. This oscillator comprises ajpentode which' has the cathode thereof connectedto ground through a resistor 151 and a regulating lamp 152. A capacitor 153 isv connected in parallel with resistor 151. The anode of pentode 150 is connected to terminal 83 through seriesconnected resistors l154 and y155'. The junction between these resistors is connected to ground through a capacitor 158. The control grid of pentode 150 is connected to ground through a resistor 156 which-is shunted by a capacitor 157. The suppressor grid of pentode 150 is connected to the cathode thereof. The screen grid of pentode 150 is connected to terminal 83 through series connected resistors 158 and 155 andv to ground through a capacitor 160. The anode of pentode 150'is connected to the control grids of a pair of parallel connected triodes 161 and `162 through a capacitor 163. The control grids of these triodes are connected to ground through a resistor 164. The cathodes of triodes 161 and 162 are connected to ground through a resistor 165 which is shunted by a capacitor 166. The anodes of triodes 161 and 162 are connected to terminal 83 through the primary winding of a transformer E171 and resistor 155. The anodes of triodes 161 and 162V are connected to the control grid of pentode 150 through capacitors 1724 and 173 and a resistor 174 which are connected in series relationship. The junction between capacitors 172 and 173 Une end terminal of the secondary winding i176 'of transformer 171 is l'connected to the center tap vof the secondary winding 177 of transformer 125. The second end terminal of transformer winding 176 is connected nto ground. The output signal from oscillator 36 is thus applied to the second input of phase detector 40. The frequency of the output signal from oscillator 36 is the same as the frequency of the signal from amplifier 38 because vchopper 35 is energized from a second output of oscillator 36.

The anodes of triodes 161 and 162 are connected to the control grid of a triode 180 through a capacitor 181 and a resistor 182 which are connected in series relationship. The control grid of triode 180 is connected to ground through a resistor 183. The cathode of triode 180 is connected to ground through a resistor 184. The anode of triodex180 is connected to terminal 83 through series connected resistors 185, 186 and 155. A capacitor 187 is connected between ground and the junction be# tween resistors and 186. The anode of triode 180 is connected to the cathode thereof through a capacitor 188 and a variable resistor 189 which are connected in series relationship. The junction between capacitor 188 and resistor 189 is connected to the control grid of a triode 191 through a capacitor 192. The control grid of triode 191 is connected to ground through a resistor 193.- The cathode of triode 191 is connected to ground through a resistor 195 which is shunted by a capacitor 196. The anode of triode v'191 is connected to terminal 83 through series-connected resistors 198 and 155. The anode of triode 191.is connected to ground through a capacitor 200 and potentiometer 201 which are connected in series relationship. The contactor of potentiometer 201 is connected to the control grids of a pair of parallel e'onnected triodes 202 and 203 through a resistor 204.- control grids of these triodes are connected to ground through ra capacitor 205. The cathodes of they two` triodes are connected to ground through a resistor 206 which is shunted by a capacitor 207. The anodes of triodes 202 vand 203 are connected to terminal `83 through the primary winding 210tof a transformer 211 and re'-v sistor 15.5; The end terminals of the secondary winding 212 of transformer 2111 are connected to respectiveterminals 213 and 2.14 which in turn are connected to coil 80 of chopper 35.` Resistors 216 and 217 are vconnected in vseries yrelationship between terminals 213 and 214. The junction between these resistors is connected to ground. Y

A portion of .the output signal from oscillator 36 is' thus' amplified to energize chopper 35. Capacitor 188,

lamp l152 through a variable resistor 175.

and resistor 18,9 provide av phase shift network which cari be adjusted so that the two signals applied to transformer 125 are" either in phase with one another or 180 out ofl phase, .depending upon 'the relative amplitudes ofthe' signal' and feedback voltages' being compared. This; rie't work is provided to compensate for any phase shift which? may result through the output chopper drive section of' oscillator 36 and throughv amplifier 38.

The' relative phase of the two signals applied to trans'- forxn'er 125' is determined by the relative amplitudes'rof the twoy signals applied tothe inputs of chopper 35. end terminals of transformer winding 177 are connected to the anod of diodes 220 and' 221 respectively. The cathodes of these diodes are connected to one another through a potentiometer 222. The contactor of potnl-j tiometer 222'is connected to ground. The cathode of diode 220 is connected tothe control grid of atriode'Z' through a resistor 224'. The; cathode of diode` 221i is" connected toV the control grid of'a triode 225 through ay resistor 226. The control-grids of triodes 223'and 225 are"- connected to ground through capacitors 227l and 228, rei spectively. Resistors 224 and 226and capacitors" 227 and 228' thus' provide filter networks for the ,outputi 'signals' from diodes 220 'and 221. The anode'of triode 223 isiorif nected to the cathode of diode 43`and 'tothe' anodeof A f7 42. The anode of diode 225 is connected to the cathode ofdiode 42 through relay coil 41 and to the anode of diode 43 throughswitch 45 and relay coil'44l Theanode of diode 223 is also connected to ar positive potential terminal 230through a resistor 231. VThe anode of diode 225 is connected to terminal'230 through a resistor 233.

The circuit is connected so that diode 42 conducts to energize coil 41 if Ithe potential applied to terminal 71 of chopper 35 is greater than' the potential applied lto terminal 75. If the potential applied totenminal 75 is greater than the potential applied to terminal 71 diode 43 conducts to energize coil 44 if switch Y45is in the downvposition.VV 5

The voltage dividingnetwork and the associated switch circuit are illustrated in Figure 3. Reference potential source 17 is applied across the end terminals of a poten'- tiometer 220. The contactor and one end terminal of potentiometer220 are'connected to the respective end terminals of voltage dividing network 15.v Potentiometer 220 thus provides an adjustable reference voltage. Stepping switch 11 comprises four switch arms 11b, 11c, 11d and 11e which are moved in unison when a driving coil 11a is energized. Switch arms 11b and 11c move so as toV positionY and'then back to the zero position'. The switchk arms move one position each'time coil 11a is'energized' Stepping'switch 12 is generally similar to switch 11 and lfor ythis reasonl is not described in detail. Itrshould 'be noted, however, that the several switch arms initially are in the -.9 position.V These switch arms movedownwardly toward lthe zero position when coil 12a is energized. The indicator lamps are designated'by letters T. Stepping switch' 13v also'is generally similar to stepping switchY 11. Switch`13. however, diiers from switch 11 in that only a single switch arm 13e is provided in conjunction with voltage dividingnetwork 27. A Y 1 Current'source 47 is connected through switch 48 to a terminal 221. Terminal 221 is-connected tothe up Vcontactof a switch `222 which is actuated -by a relay 223. Switch '222 is connected to ground through'the coil of relay 223 which is shunted by a capacitor 224. The down contact of switch 222 isconnected to the up contact otra switch 225'which is actuated by a. relay 226.' Switch 225is connected to lground through thecoil of relay 226. Terminal 221 is also'connected to switch 49 kwhich Vis actuated by negative error relay 44. YThe roY The down contact of switch 244 is' connected to ter minals 11f of stepping switch 11. The up contact of switch 51 is connected to the down contact of a switch 245 which is `actuated by arelay 246: Switch 245 is connected to the down contact of a switch 247 which is actu-ated by relay 248. p Switch 247 is connected to ground through the coil of stepping switch 13a. Switch 247 is also connectedto a switch 250 which is actuated by relay 236. The down contact of switch 250 is connected to terminals 13] of stepping switch'13.

Switch arm 11e is connected to the *,down contact of a switch 1252 which is lactuated by stepping switch coil 11a. Switch 252 is connected to the positive terminal of current source 47; Switch 12e is connected to the down contact ofv a switch 253 whichis actuated by stepping switch coil 12u. Switch 253 is connected to the'positive terminal of current source 47. Switcharmi 13e is com nected to the down contact of 4a switch 254 which is actuated by stepping switch 13a. Switch 254 is connected to the positive terminal of current source 47. Switch arms 11d, 12d and 13d are connected to the positive terminal of current source 47. Y

The up contact of switch 252 -is connected to ground through the coil Vof relay 243 which is shunted by a capacitor 256. The up Vcontact of switch 253 is connected to ground through the coil of relay 234 which is shunted by a capacitor 257. The up contact of switch 254'is, connected to ground through the coil of relay 248 which is shunted `by a capacitor 258. y

The lowermost terminals 11g, v12g and 13g of respective Astepping switches V11, 12and 13 are connected to ground through the-@coils ofrelays 241, V231Vv and 246, respectively. The positive'trminal of current source 57.- is connected to ground ythrough switch 56 and the coil of relayg236.V

This potential terminal is also connected to a switch 260 which isvactuated by relay 236. The up contact of switch260 is connected to the up contact of switch 53. Switch armvlsc'opf-stepping switch 13 is connected to the up contactof switch 37. The lowermost left han-d terminal 29 ,of voltage dividing network 27 is connected to thedown contact of switch 37 Switch 37 Vis connected nal 34 being grounded., If the stepping switches of Figure 3f areinit'ially in the illustrated positions the eedbaclc l voltage between terminals 74 and Z6 is 'equal to 99 milli# Ydown" contact of switchV 49 is connected to switch 50 f which is actuated by positive error relay 41.V The down contact` of switch 50 is connected to the down contact of switch 53. Switch 53 is connected to ground through Ithe coil of relay 54 which is shunted yby capacitor55. The .up positionof switch v49 is connected to a switch V228 which is actuated byA relay 223. The down contact of switch228 is connected to the down contact of a switch 230gwhich is actuated by a relay 2.31.V Switch 230 is connectedto the fdown contact ,of` a switch 233 which is actu-ated by a relay 234. Switchy 233 is connected Vto grpund through the coil of stepping switch 12a. Switch 233 islalso connected to a switch 235 whichrisractuated by a relay236.lk .The down contact of switch 235is connected to Vterminals 12]c of stepping switch l12.

The up` `contactof switch 5'0 is connected'to switch 51.., v'1`Vhe dovf n contact of switch 51 is connected to the down contact of a switch 240. Switch 240 is conneyctd tof'the down contact of a switch 242 which is lactuated by a relay 243. Switch 242 is connected fto ground through the coil of relay 11a. Switch' 242 is alsol onnectedto as witch244 which is actuated byrelay 236.

volts. Because the signal voltage is greater than the feed back voltage, positive'error Vrelay 41 is energized initially. Switchv 48V is closed when itis desired to measure the signal'voltage by the stepping switches. Closure of Vswitch 48 results in current llowing 'from source 47 through switches 49, 50, 51, 240 and 242 and the coil of stepping switch 11a. This results in coil 11a being energized so that the switch arms of stepping switch` 1l are moved downwardly vone position. In particular stepping switches which have been used, lthe energizing of coil lla causes a spring loaded ratchet to be set. When the coil 11a is deenergized, the arms are moved downwardly by the ratchet. Switch 252 is moved upwardly when coil 11a is energized and this results in relay 243 being energizedl from currentfsource 47. f When relay 243 is energized l switch 242 is opened so 'that coil ll'cis'deenergiz'ed. Thisy opens switch'252 to deenergizeV relay 243. However, re-f lay 243 does not deenergize immediataely Vbecause of theV charge on capacitor 256. After a predetermined interval switchj242 moves downwardly so thatA coil 11e is once again energized.' This results in Vthe switch arms ofV 9 operationto prevent the switch arms overshooting the desired value. This operation continues until the switch arms Vof stepping switch 1i are moved downwardly so as -tobe opposite Ylamp Hq. At this position the feedback voltage to chopper 35 becomes 799 millivolts which is greater than the signal voltage of 753 millivolts.

-As soon as the feedback voltage exceeds the signal voltage positive error relay 41 becomes deenergized. Current then Vi'lows from source 47 through switches 49, 50 and 53 to carry-around relay 54. When relay 54 becomes energized a circuit is completed between the coil of the relay-and source 57 through switches 53 and 26?. Carryaround relay 54 thus remains energized as long as switch 56 remains closed. It should be observed that relay 236 is energized at this time. When relay 54 is energized switch 37 moves upwardly so that the uppermost terminal 29 of voltage dividing network 27 is connected to terminal 76 in place of the lowermost left-hand terminal 29. This results in the stepback voltage being decreased to 790 millivolts. Because this feedback voltage is still greater than the signal voltage, negative error relay 44 becomes energized. This results in coil 12a being connectedin circuit with source 47 through switches 48, 49, 228, 230 and 233. The switch arms of stepping switch 12 then move downwardly in a stepwise fashion until they are opposite lamp T5. `Coils 12a and 234 cooperate with one another to provide such movement. t this final position of the switch arms the feedback voltage becomes 750 milivolts.

The signal voltage is once again greater than the feedback voltageso that the positive error relay il is again energized. This results in the coil of stepping switch 13st being connected )in circuit with source 47 through switches 48, 49, 50, 51, 245 and 247. Coils 13a and 248 cooperate with one-another so that the switch arms of stepping switch 13 are moved downwardly to a position adjacent lamp U3. This completes the stepping operation. Lamps H7, T5 and U3 are illuminated to provide a digital representation (753) of the signal voltage being measured. l

-If the signal to be measured is initially less than 99 millivolts, the error voltage is initially negative. The negative error relay 44 can not operate because relay 54 has not closed switch 45. Thus, switch 37 closes first tocause a change of 9 millivolts in Vfb. This action may cause either of relays -41 or 44 to operatae, depending upon the value of lthe signal being measured.

When it is desired to reset the mechanism, switch 56 is opened. This deenergizes relay 236. Coil lla'is connected in circuit with source 47 through switch 244, a contact 11j, switch arm 11e and switch 252. This circuit remains completed until switch arm 11e is returned to the position adjacent contact 11]". Stepping switch 12 is returned to the initial position by coil 12a being connected in circuit with source 47 through switch 253, a contact 12j, switch arm 12e and switch 235. Stepping switch 13 vis returned to the initial position by coil 13a being connected in circuit with source 47 through switch 256, a contact 137", switch arm 13e and switch 254.

Relays 241, 2-31 and 246 prevent their respective stepping switches from moving past their last positions. This breaks the circuit between coil 11a and source 47 so that stepping switch 11 cannot move beyond the lowermost illustrated position. Relays 231 and 246 serve the same functions with respect to stepping switches 12 and 13. Relays 223 and 226 eliminate the need for critical gain adjustment in the amplifier. These relays disconnect the driving voltage from coil 12a the second time positive error relay 41 is energized. This insures that the second time the error voltage becomes negative switch 21 will remain at rest.

Actuation of switches 70 and 73 of Figure 2 applies a reference voltage '7-2 to input terminals 33 and 34. Potentiometer .220-of Figure 3 can -tlren be Aadjusted if necessary until the-reading of the converter is indicative of the the signal voltage is indicated by certain lamps being illuminated. `Obviously the currents through the lamps can be directed to input circuits of a digital computer if it is desired to apply the analog Voltage to the input of such a computer. Starting switch 48 and reset switch S6 can be operated yby suitable 'timing mechanism if vit is desired to measure signal voltages at predetermined times.

While the invention has been described in conjunction with a present preferred embodiment, it should be evident that it is not limited thereto.

What is claimed is:

l. Apparatus for converting a voltage into a digital representation comprising a first voltage dividing network, a reference potential applied across said rst network, a second voltage dividing network means for connecting the end terminals of said second network to respective first and second spaced points on said first network, a third voltage dividing network means for connecting the end terminals of said third network to respective third and fourth spaced points on said second network', voltage comparing means, means for applying the voltage `to be converted to the first input of said comparing means, means for applying a voltage between a tfthpoint on said third network and one end terminal of said irstnetwork to the second input of said comparing means, and means responsive to the output of said comparing means to adjust one by one in a predetermined sequence the positions of said points on said networks until the two voltages applied to said comparing means are equal, the positions of said points being representative of the magnitude of the voltage to be converted.

2. Apparatus for converting a voltage into a digital representation comprising a first voltage dividing network, a yreference potential applied across said first network, a second voltage dividing network means for connecting the end terminals of said second network to respective first and second spaced points on said rst network, meansfor comparing voltages and having rst and second inputs,v means for applying the voltage to be converted tothe lrst input of said comparing means, means for applying a voltage between a selected one of a third point anda fourth point on said second network and one end terminal of said first network to the second input of said comparing means, one of said third and fourth points vbeing lxed, means for selecting one of said third and fourth points responsive t0 a change in sign of an output V'signal from said comparing means, and means responsive to the output of said means for comparing to adjust the positions of said points on said networks by moving the first said means and lche fourth said means in respective different directions unt-il the two voltages applied to said comparing means lare substantially equal, said respective diierent directions being the only directions in which `the respective means do move, the positions of said points being representative'of the magnitude of the voltage to be converted.

3. Apparatus for converting a voltage into a digital representation -comprising a first voltage dividing network, 'a refe-rence potential applied across said iirst network, al second voltage dividing networkmeans for connecting the end terminals of said second network to respective first and second spaced points on said first network, a third vvoltage dividing network means for connecting the end'te'rmi'nals of said third network to respective third and fourth spaced Apoints Aon said second network, voltage comparing means, mea'ns for applying the voltage to be converted to the first input of said comparing means, means for A'app'alyi'ng a voltage 'between a fth point on said third network and one end terminal of said first network to 'the second input of said comparing means, la first stepping switch to move said first and second points in one direction only on said first network when energized, la second stepping switch to move said third Aand fourth points in one direction only on said second network when energized, a third stepping switch to move said fifth point on said third network in one direction only when energized, and means responsive to the output of said comparing means to energize said stepping switches in sequence until the two voltages applied to said comparing means are substantially equal, the positions of said points being representative of the magnitude of the voltage to be converted.

4. Apparatus for converting a direct current voltage into -a digital representation comprising a first voltage dividing network, a direct current reference potential applied across said first network, a second voltage dividing network means for connecting the end terminals of said second network to respective first and second spaced points on said first network, a third voltage dividing network means for connecting the end terminals of said third network to respective third and fourth spaced points on said second network, means for comparing voltages and having first and second inputs and an output, means for applying the voltage to be converted to the first input of said comparing means, means for applying a voltage between Ia fifth point on said third net- 'work and one end terminal of said first network to the second input of said comparing means, a rst stepping switch to move said first yand second points on said first network when energized, a second stepping switch to move said third and fourth points on said second net-V work when energized, a third stepping switch to move said fifth point on said third network when energized,

and means responsive to the output of said comparing means to energize said stepping switches in sequence until the two voltages applied to said means for comparing are substantially equal, the positions of said points being representative of the magnitude of the voltage to be converted. 'Y 5. VApparatus for converting a direct current voltage into 'afdig'ital representation comprising a first voltage dividing network, a direct current reference potential applied across said first network, a second voltage dividing network means for connecting the end terminals of said second network to respective first andsecond spaced points on said first network, a third voltage dividing network means for connecting the end terminals of said third networkto respective third and fourth spaced points on said second network, means for comparing voltages and having first and second inputs and an output, means for applying vthe voltage to be converted to the first input of said comparing means, means for applying a voltage between afifth point on said third network and one end terminal of said first network to the second input of said comparing means, a first stepping switch .to move said first `and second points in one direction only on said first network when energized, a second stepping switch to move said third and fourth points in one direction only on said second network when energized, a third stepping switch to move said fifth point on said third network in one directiononly when energized, and means responsive to the output of said comparing means to energize saidV first stepping switch until the voltage applied'to said first input is not less than the voltage applied to said second input and then to energize said second stepping switch until the voltage applied to said second input is no greater than the voltage applied to said first input and then to energize said third stepping switch until the voltages applied tov said first and second inputs arersubstantially equal, the positions of said points being representative of the magnitude of the voltage to be converted.

. y6. The combination'inccordance with claim 3 further comprising means to render said first and second stepping switches inoperative following movement thereof responsive to a difference in the voltages being compared.

7. 'Ilhe combination in accordance with claim 3 further comprising means to delay the stepping of said stepping switches predetermined times to prevent movement of the points on said networks past the balance positions. 8. The combination in accordance with claim l further comprising a plurality of indicating'means associated with cachv of said networks, and means to energize respective ones of said indicating means responsive to the positions of one of said points one each of said networks.

9. The combination in accordance with claim 1 further comprising switching means to move the points on said networks to initial positions.

' l0. Apparatus for converting a direct current voltage into a digital representation comprising a first voltage dividing network, a direct current reference potential applied across said first network, a second voltage dividing network means for connecting the end terminals of said second network to respective first and second spaced points on said first network, a third voltage dividing network means for connecting the end terminals of said third network to respective third and fourth spaced ypoints on said second network, a voltage comparing means having first and second inputs and further comprising a vibrator which is adapted to engage said first and second inputs alternately, an oscillator to drive said vibrator at a predetermined frequency, an amplifier having the input thereof connected to said vibrator, a phase detector, and means for applying the outputs of said amplifier and said oscillator to the inputs of said phase detector, the polarity of the output of said phase detector being representative of the relative magnitudes of the voltages applied to said first and second inputs means for applying the voltage to be converted to said first input of said comparing means, means for applying a voltage between a fifth point on said third network and one end terminal of said first network to said second input of said comparing means, Ya first stepping switch to move said first and second points on said first network when energized, a second stepping switch to move said third and fourth points on said second network when energized, a third stepping switch to move said fifth point on said third network when energized, and means responsive to the output ofr said comparing means to energize said first stepping switch until the'volta'ge applied to said first input is no less than the voltage applied to said second input and then to energize said-second stepping switch until the voltage applied `to said second input is nogreater than the voltage applied to said rst input and then to energize said third stepping switch until the voltages applied to said first and second inputs are substantially equal, the positions of said points being representative of the magnitude of the voltage to be converted.

ll. Apparatus for coverting a direct current voltage into a digital representation comprising a firstvoltage dividing network, a direct current reference potential applied across said first network, a second voltage dividing network means for connecting the end terminals of said second network to respective first and second spaced points on said first network, a third voltage dividing net-kv work means for connecting the end terminals of said third network to respective third and fourth spaced points on said second network, a voltage comparing means having first and second inputs and further Vcomprising a vibrator which is adapted to engage said first and second inputs alternately, an oscillator to drive said vibrator at a predetermined frequency, an ampler for providing output signals of substantially constant amplitude when the'input signals applied thereto are greater than apredeterrnined value, said amplifier having the input thereof connected to said vibrator, a phase detector, and means for applying the outputs of said amplifier and said oscillator to the inputs of said phase detector, the polarity of the output of said phase detector being representative of the relative magnitudes of the voltages applied to said first and second inputs, means for applying the voltage to be converted to said first input of said comparing means, means applying a voltage between a fifth point on said third network and one end terminal of said first network to said second input of said comparing means, a first stepping switch to move said first and second points on said first network when energized, a second stepping switch to move said third and fourth points on said second network when energized, a third stepping switch to move said fifth point on said third network when energized, and means responsive -to the output of said comparing means to energize said first stepping switch until the voltage applied to said first input is not less than the voltage applied to said second input and then to energize said second stepping switch until the voltage applied to said second input is no greater than the voltage applied to said first input and then to energize said third stepping switch until the voltages applied to said first and second inputs are substantially equal, the positions of said points being representative ofthe magnitude of the voltage to be converted.

12. Apparatus for converting a direct current voltage into a digital representation comprising a first voltage dividing network, a direct current reference potential applied across said first network, a second voltage dividing network means for connecting the end terminals of said second network to lrespective first and second spaced points on said first network, a third voltage dividing network means for connecting the end terminals of said third network to respective third and fourth spaced points on said second network, voltage comparing means, means for applying the voltage to be converted to the first input of said comparing means, means for applying a voltage between a fifth point on said third network and one end terminal of said first network to the second input of said comparing means, a rst stepping switch to move said first and second points on said first network when energized, a second stepping switch to move said third and fourth points on said second network when energized, a third stepping switch to move said fifth point on said third network when energized, and means yresponsive to the output of said comparing means to energize sa-id first stepping switch until the voltage applied to said first input is not less than the voltage applied to said second input and then to energize said second stepping switch until the voltage applied to said second input is no greater than the voltage lapplied to said first input and then to energize said third stepping switch until the voltages applied to said first and second inputs are substantially equal, the positions of said points being representative of the magnitude of the voltage to be converted, the last said means comprising first and second relay coils, means to energize said first coil representation comprising a first voltage dividing network,

a reference potential applied across said first network, a second voltage dividing network means for connecting the end terminals of said second network to respective first and second spaced points on said first network, a third Voltage dividing network means for connecting the end n terminals of said third network to respective third and fourth spaced points on said second network, voltage comparing means, means for applying the voltage to be converted to the first input of said comparing means, means for applying a voltage between a fifth point on said third network and one end terminal of said first network to the second input of said comparing means, a first stepping switch to move said first and second points on said first network when energized, a second stepping switch to move said third and fourth points on said second network when energized, a third stepping switch to move said fifth point on said third network when energized, a first coil associated with each of said stepping switches for energizing said stepping switches, means to delay the stepping of said stepping switches predetermined times to prevent movement of the points on said networks past the balance positions, said means to delay comprising a second coil associated with each of said switches, a first switch which is closed responsive to said first coil being energized, a second switch lwhich is opened responsive to said second coil being energized means for applying current to said first coil through said second switch, and means for applying cur-rent to said second coil through said first switch, and means responsive to the output of said comparing means to energize said stepping switches in sequence until the two voltages applied to said comparing means are substantially equal, the positions of said points being representative of the magnitude of the voltage to be converted.

References Cited in the file of this patent UNITED STATES PATENTS 2,497,961 Shaw Feb. 21, 1950 2,508,424 Rouy May 23, 1950 2,625,822 Nichols Ian. 20, 1953 2,775,754 Sink Dec. 25, 1956 2,896,198 Bennett July 21, 1959 

